[ovirt-users] [ovirt-devel] [SCALE][RFC] ksmd bound to one core

[Michal Skrivanek]

On Nov 5, 2014, at 23:29 , Daniel Helgenberger <daniel.helgenberger at m-box.de> wrote:

> 
> 
> On 05.11.2014 21:07, Adam Litke wrote:
>> On 04/11/14 13:11 +0100, Sven Kieske wrote:
>>> Hi,
>>> 
>>> currently ksmd is a single process
>>> and is thus bound to one core.
>>> 
>>> This leads to some scaling problems such as:
>>> 
>>> If you got a lot of vms on one host with huge amounts
>>> of ram you can observe that the cpu usage by ksmd
>>> goes easily to 100%.
> I wonder what would be the benefit here... I think spending CPU cycles 
> on something like memory compression is not what (most) users would do. 


Indeed.
I would say it is more beneficial to spend time on better analysis of what regions of memory you want to analyze. Instead of this brute force approach trying to scan the whole hosts memory aggressively. 
E.g. We don't even have ksm per VM yet (but it's in the plan). Even that would help a lot as you can then enable it only where it makes sense (pool deployment with many same/idle VMs)

Thanks,
michal

> Already I think this is an annoyance; maybe thats why the process is 
> niced to +5. A multi process daemon would require careful confining 
> (with cgroups).
> To my understanding the for KSM to really work well is to have many 
> (idle / high mem) guests which are quite similar?
> 
>>> 
>>> I wonder if ksmd could not be split up
>>> in child/worker threads, thus enabling higher density
>>> of vms on one host.
>> 
>> It's likely going to be trickier than you imagine with the added
>> locking that would be required to synchronize the ksmd threads.
> Actually I was refraining to answer Svens question since I lack some 
> knowledge here. But I guessed synchronization was the reason for the 
> single threadted design.
> Some serous work needs to be done on KSM do achieve this goal. I was 
> thinking about some map/reduce aglo to do that... OTOH mangeling with 
> mem pages of guests very requires special care.
>> 
>>> 
>>> or can this just be tweaked by altering values in
>>> /etc/ksmtuned.conf ?
>> 
>> I don't think it can.
>> 
>>> What do you think?
>> 
>> Interesting idea.
>> 
> 
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